Question: The scale on a map is 10cm : 10km. If two cities are 20cm apart on the map, what is the actual distance in km between the two cities?
Solution: The scale means that every 10cm on the map corresponds to an actual distance of 10km. A distance of 20cm on the map is the same as 2 $\cdot$ 10cm. The actual distance between the two cities is 2 $\cdot$ 10km, or 20km.